The purpose of this study is to clarify the mechanisms responsible for the initiation of cardiac arrhythmias and means for their prevention and treatment in pediatric patients. This is important for two reasons: first, pediatric patients with cardiac arrhythmias differ from adults in their response to pharmacologic agents; and second, there is evidence that electrophysiologic function in the hearts of pediatric patients differs from that in adults. To perform this study, we shall obtain tissues from patients (neonates, infants, and children) undergoing cardiac surgery and from neonatal and young dogs. We shall study the electrophysiologic and contractile properties and the responses to pharmacologic agents of these tissues in order to identify the mechanisms responsible for the occurrence of arrhythmias and means for their prevention and treatment. We shall also study the effects of drugs on the electrophysiologic properties of the in situ heart, and identify the relationships between plasma and tissue drug concentrations and electrophysiologic events. The drugs to be studied are digitalis, antiarrhythmic drugs, acetylcholine, and catecholamines. Standard microelectrode and catheter electrode techniques will be used for the electrophysiologic studies, Standard methods will also be employed for measuring the contractility of isolated cardiac muscle strips. As a result of these studies we will obtain information concerning the particular mechanisms responsible for the initiation of arrhythmias in the pediatric population and means whereby their prevention and treatment may be improved. Such information not only should augment our present abilities with respect to diagnosis and therapy, but should aid in the identification and testing of additional modalities for prevention and treatment of arrhythmias in the pediatric patient.